The invention relates to biocompatible implants and to metal alloys and methods for constructing biocompatible implants. In preferred embodiments, the invention relates to biocompatible joint implants and to materials and methods for constructing these implants.
The replacement of joints with man-made artificial joints, i.e., joint arthroplasty, has grown dramatically in the past several decades. Several different joint replacement systems are currently available. However, the worldwide standard remains a cobalt-base superalloy ball structure which fits into a socket formed of ultra-high molecular weight polyethylene (UHMWPE).
The cobalt-base superalloy used in joint implants, CoCrMo, is particularly desirable because of its biocompatibility, high yield strength, and high hardness. American Society for Testing and Materials (ASTM) Specification F 1537 94 defines the chemistry of this alloy as set forth below:
There is also an ISO specification covering CoCrMo, ISO 5832-12, which is identifical in chemistry and very similar otherwise to ASTM F 1537 94.
This alloy is solid solution hardened by the presence of Mo and to some extent Cr, and precipitation hardened by chromium carbides. In addition, the wrought version of the CoCrMo alloy is strengthened by work hardening. This can be accomplished by cold working (e.g. drawing or rolling at room temperature) or by warm working (e.g. rolling at relatively low temperatures within the allowable working temperature range.)
The presence of carbon, in particular, is considered critical to achieve necessary strength properties via the precipitated carbides. Although carbide strengthening is more important for the cast alloy and particularly at elevated temperatures (see Sims, C. T., Stoloff, N. S. and Hagel, W. C., The Superalloys II, pp 135-163 (John Wiley and Sons, 1987); nevertheless, even in the wrought alloy which has a relatively low carbon content (typically 0.05%), there are normally huge numbers of very small ( less than 5 xcexcm) carbide particles dispersed throughout the alloy matrix. It is also not unusual to find slightly larger particles of the intermetallic compound, sigma, present in the alloy. Sigma is a very hard and brittle compound of the general formula, Cox (Cr, Mo)y.
The CoCrMo ball structures used in joint implants are usually machined from wrought bar stock. The thermo-mechanical forging process reduces the size of the hard carbide particles (as compared to the carbides in cast balls) which increases both alloys strength and hardness, and provides a corresponding reduction in surface roughness of the ball; Streicher, R. M., Tribology of Artificial Joints, Endoprosthetics, Morscher, E. W. (Ed.), Berlin:Springer, p. 38-48 (1995.). More specifically, the wrought alloy has a fine grain size (ASTM 5 or finer), a high strength (120 Ksi min. Y.S.), and high hardness (Rc35 typical).
Polyethylene wear remains the limiting factor for the longevity of joint arthroplasty. Wear rates on the order of 0.09 mm to 0.3 mm per year have been reported. As a result of such wear, submicron particles are released from the joint at a rate of on the order of 40 billion particles per year.
Whether or not the debris cause an immediate clinical problem depends upon the body""s response to the particulate wear debris. Nevertheless, the polyethylene wear particles can have long term effects of bone loss and loosening of the implant. In particular, the wear debris can overload the afferent transport system leading to accumulation of debris around the articulation. A soft tissue membrane forms as a result of the biological reaction to the debris producing soluble factors that stimulate bone resorption, causing osteolysis and loosening of the implant.
The effect of polyethylene wear debris as a primary cause of long term joint implant failure has been known for over two decades and has generated widespread efforts to develop new joint implant structures and materials to reduce wear debris. Substantial effort has been focused on improving the polymers used to form the cup or socket portion of the artificial joint to directly minimize polyethylene debris. Such proposals have included improved sterilization and polymer hardening techniques.
Proposals for improving the ball structure to reduce wear debris in artificial joint implants have focused on frictional properties of the ball surface, and on hardness of the ball alloy. Hardness properties are significant because of third body wear which occurs when particles become trapped between two articulating surfaces. The presence of bone, cement and/or metal debris are believed responsible for roughening of the ball surface in artificial joint implants, causing in turn, increased abrasive, two body wear as the roughened ball grates across the softer polyethylene. Indeed, the tendency of titanium alloy hip prostheses, which were used in the 1970""s, to cause rapid polyethylene wear was due to titanium""s susceptibility to oxidative and third body wear. This experience led to acceptance of the harder and stronger CoCrMo alloys as the xe2x80x9cgold standardxe2x80x9d, Cukler et al., Femoral Head Technologies To Reduce Polyethylene Wear In Total Hip Arthroplasty, Clinical Orthopaedics and Related Research, No. 317, pp. 57-63 (1995).
Despite the high hardness values associated with CoCrMo alloys used in joint implants, scratched and pitted surfaces are also seen in studies of balls of these alloys recovered from patients after use. This has led to proposals for improving surface hardness of the CoCrMo alloy ball. For example, U.S. Pat. No. 5,308,412 to Shetty et al. proposes nitrogen ion implantation to enhance the hardness of the surface of a cobalt-chromium implant. Similarly, Streicher, (cited above), reported improved wear resulting from a TiN coating on cast CoCrMo femoral parts. Alternative ball constructions based on ceramics, particularly zirconia and alumina, have also been investigated because of the extremely high hardness values associated with these materials as disclosed in U.S. Pat. No. 5,180,394 to Davidson; U.S. Pat. No. 3,871,031 to Boutin; and Cooper et al., Ceramic Bearing Surfaces In Total Artificial Joints: Resistance to Third Body Wear Damage From Bone Cement Particles, Journal of Medical Engineering and Technology, Vol. 15, No. 2, pp. 63-67 (1991).
Various efforts and proposals have also been made to improve frictional characteristics of the surfaces of artificial implants. As noted previously, the smaller size of hard carbide particles on the surface of the current standard wrought CoCrMo alloy balls provide reduced surface roughness. In particular, reduction in size of the carbides from a diameter of 20 xcexcm (cast alloy) to 2-3 xcexcm (wrought alloy) reduced polyethylene wear by 20%, (Streicher, cited above). Gold et al., Metal-On-Plastic Total Hip Joints, Clinical Orthopaedics and Related Research, No. 100, pp. 270-278 (1974) suggest a surface roughness less than four microinch (0.1 xcexcm) but greater than 2 micro-inch would be ideal in a metal ball/plastic cup joint implant. Nishimura et al., Modification of The Frictional Surfaces Of Artificial Joints, Journal of the American Society for Artificial Internal Organs, Vol. 39, No. 3, pp. M762-M766 (1993) propose patterned surfaces to improve the frictional characteristics of the articulating surfaces of artificial joints. The pebbled and dimpled patterns can provide reservoirs for lubrication fluids and also provide possible sites for trapping otherwise deleterious wear particles.
Despite numerous studies and research extending over many years, polyethylene wear debris generation in artificial joints remains a significant problem and a substantial cause of long term implant failure. Although numerous modifications and alternatives have been proposed for CoCrMo alloys, the current wrought alloys continue to be the material of choice, particularly for construction of articulating joint surfaces, despite the surface deterioration that occurs over periods of long term use of these alloys.
The invention provides improved biocompatible implant alloys and methods of constructing artificial implants having improved long term wear properties. Biocompatible implant alloys provided according to the invention can have hardness and strength properties comparable to or greater than the standard CoCrMo alloy, with significantly improved fatigue life and superior wear properties with UHMWPE. Artificial implant constructions and methods provided according to another aspect of the invention are capable of eliminating latent defects that can promote long term failure of joint implants.
According to a first aspect of the invention, artificial implant components are formed of a biocompatible metal alloy having a hardness greater than about 40 Rc, a yield strength greater than about 120, and a grain size finer than ASTM 10, and which is essentially free of carbide, nitride, and sigma particles. Preferably, the alloy is an essentially single-phase alloy, i.e., the alloy is essentially free of all second phase particles.
In one advantageous embodiment of the invention, the biocompatible implant alloy is formed of a cobalt-base alloy, preferably a CoCrMo alloy, which is substantially free of carbide, nitride, and sigma second phase particles. In accordance with this aspect of the invention, it has been found that second phase particles can be essentially eliminated in CoCrMo alloys while maintaining biocompatibility, high strength and high hardness values equivalent to or exceeding conventional CoCrMo implant alloys. Moreover, it has been found that elimination of the carbide, nitride and sigma phase particles from CoCrMo alloys can substantially improve the wear properties and fatigue properties of the alloy.
While not wishing to be bound by theory, it has been found that second phase particles on or in the surface of conventional CoCrMo alloy implant structures can detach from, or out of, the alloy surface during accelerated aging in a saline environment, in the complete absence of abrasive contact on the surfaces of the implants. Although the exact cause for separation of the second phase particles is not fully understood, it is believed that separation occurs because the second phase particles have chemical and electrical properties that are different from the remainder of the cobalt-base alloy. In turn, the different electrical and chemical properties of the different portions of the alloy can lead to galvanic corrosion, and/or differential chemical erosion of the alloy surface. Irrespective of the mechanism or cause for detachment of the second phase particles from the main alloys, it is now apparent that these particles are highly susceptible to loosening and removal during the long-term residence time of an implant within the saline and abrasive environment of the body. In turn, the hard second phase particles and the pitted ball surface can generate substantial abrasive wear of a joint implant ball surface, in vivo.
The single phase, high hardness, high strength, biocompatible alloys provided according to the invention are particularly desirable for use in the bearing, i.e., articulating, surfaces of implant components in artificial hip, knee, shoulder, ankle, elbow and other joint implants because they do not generate detached second phase particles, and accordingly minimize third body wear during long term use.
The alloys of the invention are also desirably used to form implant components and fixation structures that are in a juxta-articular position to other implant components, since release of second phase particles may otherwise enhance loosening of implants via various biological responses. In addition, the biocompatible alloys of the invention are also desirably used to form non-articulating implant and fixation devices, and non-articulating components of articulating implant structures. Thus, the alloys of the invention are also desirably used to form various implant stems, base plates and the like, including, for example, acetabular and femoral components of hip replacement implants; tibial and femoral components of knee replacement implants; tibial and femoral stems of knee replacements components of shoulder replacement implants; and non-articulating components of ankle and elbow replacement implants. Similarly, the alloys of the invention are also advantageously used to form fracture fixation devices and components such as nails, screws, and plates.
In general, the substantial absence of second phase particles minimizes the possibility of chemical changes in body fluids and tissues that might result from the electrical and chemical activity associated with in vivo galvanic corrosion, or from the released particles, themselves, during long term use of the implant. Also the improved fatigue strength of preferred alloys of the invention provides longer life and a reduced chance of fatigue failure. Moreover, the improved fatigue properties provide the ability to vary design parameters such as shape and cross-section of implant devices.
In preferred embodiments of the invention, high strength biocompatible alloys essentially free of carbide, nitride and sigma particles are provided having a composition consisting essentially of about 26 to about 28 weight percent chromium, about 5 to about 6 weight percent molybdenum, up to about 1 weight percent manganese, up to about 1 percent nickel, up to about 0.75 weight per iron, up to about 0.07 percent by weight carbon, up to about 0.25 weight percent nitrogen, less than about 0.10% Si, less than about 0.02% Ti, the remainder of the alloy constituting cobalt and impurities. These implant alloys can be provided in a cast or forged form. Preferably, the alloy is in wrought form and has a hardness of about 35 Rc, preferably at least about 40 Rc, and a yield strength of at least about 120 Ksi, more preferably at least about 130 Ksi. Alloy compositions according to the present invention within these ranges meet all existing chemical requirements of ISO 5832-12 and ASTM F 1537 94. Preferably, the combination of alloy components provided according to the present invention are selected to provide a composition in the form of warm rolled and air cooled bar in which all elements are dissolved within the alloy and there are essentially no second phase particles.
According to a second aspect of the invention, machined artificial implants and implant components of improved long-term wear and durability are provided which are substantially free of latent machining surface defects or damage. In accordance with this aspect of the invention, shaped surfaces of Cobalt-base alloy implants and implant components are prepared using a precision cutting or metal removal operation, which does not leave a thin layer of deformed or damaged material, or otherwise deform the metal alloy.
It has now been found that mirror-smooth surfaces on commercially available balls of femoral implants contain latent, sub-surface scratches, gouges, and rough areas that can be xe2x80x9cdevelopedxe2x80x9d by exposing the balls to non-abrasive accelerated aging in a saline environment, i.e., accelerated aging in the absence of abrasive contact. More surprising, it has been found that the types and numbers of defects developed during non-abrasive, accelerated aging of unused femoral balls substantially match the defects seen on balls recovered during revision of failed conventional total hip arthroplasties (metal to plastic articulation). It is now believed that the traditional grinding and polishing steps used to prepare articulating implant surfaces generate a polished alloy layer that conceals scratches, and other rough areas formed during an early stage of the manufacturing process. Although various types of subsurface damage of alloy structures by machining operations have been observed before in other technological fields,. surface defects on articulating surfaces of alloy implants have been thought due to abrasive contact, in vivo. Despite substantial study and numerous investigations extending over many years, it has not been previously suggested that conventional grinding and polishing might constitute a substantial cause of surface deterioration, in vivo, of articulating surfaces of cobalt-base alloy implants.